In the disclosure of the present invention reference is mostly made to the treatment of diabetes, however, this is only an exemplary use of the present invention.
A general type of drug delivery devices suitable for delivery of a user set amount of drug comprises a spring which is strained during dose setting, the stored energy subsequently being used to expel the set dose of drug from a cartridge arranged in the device. The user usually strains a spring by rotating a rotatable dose setting member, the force thereby applied by the user being stored in the spring for later release. This type of drug delivery device may be provided either in the form of a pre-filled disposable device or in the form of a durable device adapted to be loaded with a drug cartridge by the user.
An example of a known “wind-up” drug delivery device having a pen-formed configuration and applying a torsion spring is disclosed in U.S. Pat. No. 5,104,380. In this wind-up device, or “auto-pen”, the dose setting member is located at the proximal end and works such that when the user rotates the dose setting member the spring is strained and maintained in this strained position until the user releases the set dose by activating the latch provided on the side of the housing. WO 2012/128699 discloses a “wind-up” drug delivery device having means for setting a desired limit for the size of dose which can be set. The wind-up pens disclosed in U.S. Pat. No. 5,104,380 and WO 2012/128699 have the disadvantage that if a user sets a dose too large it is not possible to decrease the set dose. The user then has to release the latch mechanism thereby expelling the entire set dose before a new correct dose can be set and delivered.
Addressing this problem, wind-up pens in which the user can actually decrease the set dose prior to dosing has been proposed, see e.g. WO 2006/045526 and WO 2010/089418.
These “automatic” delivery devices are based on a spring which is tightened during dose setting and thereafter released to inject the set dose. If a user erroneously sets a dose higher than needed these injection devices has the possibility of lowering the set dose by rotating the dose setting member in an opposite rotational direction. Such dial-down mechanisms can therefore save the user from expelling expensive drug due to an erroneous dose setting.
In WO 2006/045526, the dial-up/dial-down mechanism is based on a flexible ratchet arm which is locked in a one-way engagement with a toothed ring. When the user sets a dose the dose setting button provided at the proximal end of the delivery device is rotated. This dose setting button is connected to the ratchet element via a longitudinal stretching tubular sleeve. The ratchet element is provided with a ratchet arm in a toothed engagement with the toothed ring such that the ratchet arm when the dose setting button is rotated locks against the force of the torsion spring in the subsequent teeth of the toothed ring thereby straining the torsion spring in incremental steps. In order to reduce the set size, the ratchet arm is actively pulled out of engagement with the toothed ring whereby the force accumulated in the torsion spring rotates the ratchet element rapidly backwards such that the ratchet arm engages the previous tooth in the toothed ring thereby lowering the set dose with one increment. In this way a releasable one-way ratchet mechanism is provided allowing a set dose to be diminished. The FlexTouch® and FlexPro® drug delivery devices provided by Novo Nordisk, Bagsværd, Denmark comprise a ratchet mechanism of the type disclosed in WO 2006/045526. WO 2011/025448 discloses a further drug delivery device comprising a ratchet mechanism of this type.
The dial-down arrangement known from WO 2006/045526 could be referred to as being an “active” dial-down arrangement as the ratchet arm needs to be radially and actively moved free of its toothed engagement in order to dial down the set dose size. An example of a “passive” dial-down arrangement is known from e.g. WO 2008/031235 disclosing a dose setting mechanism with a two-way ratchet.
As an alternative to the wind-up type of automatic drug delivery devices, devices have been proposed which are provided with a pre-strained spring in which sufficient energy is stored for expelling the entire dispensable amount of drug contained in a cartridge, e.g. 3 ml. The dose setting means will typically be similar to the above-described dial-up/dial-down arrangements allowing a user to set and adjust a dose to be expelled.
Having regard to the above, it is an object of the present invention to provide a drug delivery device of the automatic type which is simple and reliable and allows for cost-effective manufacturing. The device may comprise a resettable dose setting mechanism, e.g. of the above-described “active” type.